david richerson modern ceramic engineering.pdf

7/26/2019 David Richerson Modern Ceramic Engineering.pdf

1/844

Properties Processing and

Use

n

Design

Second

Edition

Revised and Expanded


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Contents

Prel." to the Second Edilion

Preface to the First Edition

Introd.dion

Pad

I STRtlCfllRES

AND PROPERTIES

1 Atomic: Bonding, and Crystal Structure

vii

xi

I

3

2 Crystal Chemistry

and

Speci6c

Crystal

Structures 32

3 Phase EguUibria and

Phue

Equilibrium Diagrams

71

4 PbysicaJ and }bennal Behavior

123

Mrtblola Bcbuior and Measurement 162

6

EI.dr i r l

Behayio[

204

1 Dieledric

t

Magnetic. and Optical Behavior

1S1

8 Time, Temperature, and Environmental Elred

on PropeJ1Jes

313

Part II PROCF,sSING OF CERAMICS

7

9 Powder Processing

10 ShapeFonnlllR Processes

J

1

D nqf in l inn

12 Final Macbining

13

Quality

Assurance

pad II DESIGN WITH

CERAMICS

14 DesI n Considerations

15

Deslp

Approaches

6

FaOure

Analysis

17 TougbeDing of Ceramics

18 AppUalions: Material Selection

Glossary

EWed:ive

Ionic

Radii (or CalioD'

aod

AniOBS

periodic

Table

of

the

Elements

lode

x

374

418

519

5%

ZO

649

651

662

680

731

808

833

843

851


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4

Chapter 1

The

second

shell

has eight electron

s. two

in s o

rbital

s and

six in orb itals.

All h

ave

higher energy than

th

e two electrons

in th

e

first

she

ll

and are in

orbitals farther from the nucleus For

in

stance the s orbitals of the second

shell of lithium have a spherical probability distribution at about 3

A

radius.

he

p orb

it

als are not sphe

ri

cal. but have dumbbell-shaped probability di s

tributions along the orthogonal axes

as

shown in Fi

g.

1.1. These electrons

have

sl

ightly

hi

gh

er

energy than

s

electrons of the same she

ll

and are

in

pairs

with oppos

it

e sp

in

s along each ax is when the she ll is full.

he

third quantum shell has d orbitals

in

addition to sand

p

orbital

s

A

f

ull

d orbital conta

in

s

10

electrons. he fourth and fifth she lls contain f orbitals

in

addition to

s

p and d orbitals. A

full

f orbital contains

14

e lectrons.

A s

imple notation

isused

to

show the electron configuration swithin s

hell

s.

to show the relative energy of the electrons and thus

to

s

how

the order

in

which th

e electrons can

be added to or removed

from ana

tom during bond

ing

This notation can best be illu st rated by a few examples

Example 1 1 Oxygen

ha

s eight e lectrons and h

as the

electron notation

Is 2s 2p .

h

e I and 2 preceding the

nd

p designate the quantum shell.

the

nd

p designate the subshe ll wi thin each quantum she ll . and the s

u-

perscripts

designate the total

number of

electrons in

each

s

ub

shell. For oxygen

the Is a

nd

2s subshells are both full . but the 2p subshell is two electrons short

of being full.

Example 1 2 As

the

a

tomi

c

number and

th

e

number

of electrons

increase

the energy

difference

between electrons

and

be tween s

hell

s decreases and

over

lp

between quantum

gro

ups

occurs.

For example the 45 subs

hell

o iron

lill

s before the

d

subshe ll is full. This is shown

in

the electron notation by

Figure 1 1 Elec tron probability distributions for p orbital

s. he

hi ghes t probability

electron

pos itions are along

the

orthogon

al

axes wo electrons

each

with opposite

spin. are associated

with

each axis.

resulting

in a to tal of

six

el

ect

rons if

all th

e

orbitals in th e shell are filled


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9

Chapter 3

.

,

.

Figure 3.18 Transmission electron micrograph showing

an

example o liquid

im-

miscibility. Courtesy of D Uhlmann. University of Arizona .)

olymorphism

Polymorphic transformations are also shown on phase equilibrium dia-

grams Figure 3 20a is a schematic of a binary eutectic diagram with no

solid solution and with three different polymorphs o the A composition

he different polymorphs are usually designated

by

letters of the greek

alphabet. Figure 3.20b

is

a schematic

o

a binary eutectic diagram with

three A polymorphs. each with partial solid solution of

B

Figure 3.21 illustrates a real binary system with polymorphs. Poly

morphic

transformations are

also

present in Fig

3 19

Three-Component Systems

A three-component system is referred

to

as a

terti ry

sysfem The addition

o a third component increases the complexity o the system and o the

phase equilibrium diagram. he phase rule becomes = 3 - P 2 =

5 - P

As

with binary ceramic

systems. diagrams

are usually drawn with

pressure as a constant condensed system). he phase rule for the con-


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174

Chapter 5

Figure

5 S

Scanning electron photomicrographs of fracture surfaces of reaction-

bonded silicon nitride containing nearly spherical pores resulting from air entrap-

ment during processing Arrows outline flaw dimensions used to calculate fracture

stress


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178

Chapter 5

< ,

1

Figure 5.7 Typical ceramic tensile test specimen configuration.

Another method of

obtaining tensile strength

of

a ceramic material s

known as the theta test [18]. The configuration

s

shown n Fig. 5.6c.

Applicaton

of

a compressive load to

the

two arches

produces

a uniaxial

tensile stress n the crossbeam. Very little testing has been conducted with

this configuration owing largely

to

difficulty

n

specimen fabrication.

ompressive Strength

Compressive strength

s the

crushing strength of a material as shown n

Fig. 5.6f.

t

s

rarely measured for metals . but

s

commonly

measured

for

ceramics. especially those

that

must support structural loads. such as re-

fractory brick or building brick. Because the compressive strength of a

ceramic material s usually much higher than the tensile strength t s often

beneficial to design a ceramic

component

so that it supports heavy loads

n compression rather than tension . In fact. n some applications the ce-

ramic material s prestressed in a state of compression to give it increased

resistance to tensile loads that will be imposed during service. The residual

compressive stresses must first be overcome by tensile stresses

before

ad-

ditional tensile stress can build up to break the ceramic Concrete pre-

stressed with steel bars

s

one

example . Safety glass

s

another

example.


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192

Chapter 5

crew dislocation

Figure 5.16 Simple schematic

ill

ust rating a screw dislocation.

From

Ref. 7. p.

92 . )

the structure is

distorted and

und

er

localized s

tr

ess even when the overall

material is not under an app

li

ed stress. This residual stress state can be

visua

li

zed y examining Fig. 5.17.

The

dislocation ]jne extends into the

structure perpendicular

to

the

su

rf

ace

of

the

page. Note that the

str

ucture

is

distorted so as to

fill in

the space

of

the missing half-plane of atom

s

This results in a state of residual tens ile stress just below the ext ra plane

of

atoms ba lanced y compressive stress

in

the region above

th

e di slocation.

The presence

of

the disloca

ti

ons and the associated residual st

re ss

allows slip

to

occur a long atom planes at a fraction of the 2 value

th

at

Zone of compressive stress

Zone of

t nsil str ss

E

Figure 5.17 Schematic of the residual st ress state showing compressive stress

above the dislocation and tensile stress below the dislocation. I ASM

In

te rna

tional. )


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igure5.23 Crystal structure of A I ~ o \ showing complex paths O

-

a

nd

Alh ions must follow to allow slip to occur under an

applied stress From

W

D. Kingery et

al lntroduction

o

Ceramics

2nd ed .. Wil ey. New York.

1976

.

p

732.)

;

.

=

'

.

=

;

3

-

g


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Electrical Behavior

43

Figure 6.23 Example

of

the Meissner effect showing the levitation

of

a magnet

at liquid nitrogen temperature by YBa u .t01., ceramic superconductor. (Courtesy

Ceramatec. Inc .)

The response of the superconductive material to the amount of current

being carried

or

to an applied magnetic field is also very important. Too

high a current density or magnetic field can destroy the superconductive

behavior. Each material has a different response.

Evolution of Superconductor Materials

Figure 6.24 shows the historical progression in discovery o super ondu tive

materials with higher T . Progress was extremely slow up to 1986, averaging

about 4 K per decade . Initial materials identified to be superconductive

were elemental metals (Hg, Pb, Nb), followed primarily

by

solid solutions

(NbTi) and intermetallics (Nb,Sn , V,

Si

,

Nb

,G e). Until the early 1960's,

relatively few materials had been identified with superconductive behavior.

Superconductivity was thought to be

an

anomalous property . Since

96

techniques have been avai lable to achieve temperatures closer to absolute

zero (on the order of 0.0002

K

a

nd

to simultaneously apply high pressure.

Under these conditions many more elements, so lid solutions, intermetal

lics, and ceramics have been demonstrated to have superconductivity.

Several ceramic compositions were identified to be superconductive.

These included tungsten, molybdenum, and rhenium

"bronze" composi

tions A,WO A,MoO .. and A,RhO where A was Na,

K,

b , Cs, NH

Ca, Sr, Ba, etc.; oxygen-deficient SrTiO

J

and LiTi0

3

; and BaPb, _Bi O

J

.


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Dieleclric. Magnelic Optical Behavior

275

equal probability

of

shifting

in

s

ix

directions toward

one

of

the

corners

of

the

octahedron.

As

a result. the tetragonal crystal contains

some

dipoles

in one

portion

of

the crystal pointing

in

one direction whereas others

in

another portion may point

in

a direction

9

0

or

18

0

away from the first.

A region of the crystal

in

which the dipoles are aligned

in

a common

direction is called a

domain.

An example of

BaTiO

.

1

with a ferroelectric

domain with aligned dipoles is illustrated in Fig. 7.18.

Le t us return now to Fig. 7.16 and describe what happens

in

a ferroe-

lectric crystal such as tetragonal BaTiO, when an electric field is applied.

The ferroelectric domains are randomly oriented prior to application of

the electric field, that is, at

E 0,

the net polarization equals zero

P,,, 0).

As we apply an electric field and increase the electric field, the domains

begin to move in the

aTiO

.\

and

align parallel to the applied field . This

results

in an

increase

in

net polarization along line OA. The polarization

reache s a saturation value

8)

when all the domains are aligned

in

the

direction

of

the field. If

we

now redu

ce

the electric field to zero many

of

the domains will remain aligned

such

that a

remanent polarization P,)

exists.

Interpolation

of

the line

8e

until it intersects the polarization

axis

gives a value P

J

which is referred

to as

the spontaneous polarization. f

we

now reverse

the

electric field

we

force domains to begin to switch

direction. When enough domains switch the domains

in

one direction

balance

the

domains in the opposite direction

and

result in zero net

po

Figure 7.18 TEM image of

180

0

ferroelectric domain

s

in

a

single grain of

BaTiO,. Courtesy of W. E Lee, University of Sheffield.)


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Dielectric , Magnetic , Oplical Behavior

85

Another importa

nt

wave-generation application is the sonic delay line.

A delay line consists of a so lid b r or rod of a sound-transmitting material

glass , ce ram ic, metal) with a transduce r attached to each end. An electric

signal that is to be delayed is input to the first transdu

ce

r. The signal

is

converted to a sonic wave impulse that travels along the sound-transmitting

waveguide. The sonic impulse is

th

en converted back to

n

electrical

impulse by the second transducer. The delay results because a sonic wave

travels much more slow ly than elec trons passing through a wire . The time

of delay is controlled by the length of the waveguide. Delay lines are used

extensively in milit ry electronics gear a

nd

in color te levision sets. One

exa

mpl

e

is

radar systems to co

mp

a

re

informa

ti

o n from one echo with the

next echo and for range calibration.

The wave-generation applications

di

scussed

so

f r

involve acoustic

waves transmitted through bulk media. Additional freedom exists

in

the

Figure 7.25 Piezoelectric ceramics nd assemb es for a variety of applications.

(Co

urt

esy E O Corporation.)


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324

Chapler 8

-

.

Figure 8.7 Hot pressed specimen deformed by creep under a load of 276

MPa

40.000 ps

i)

at

llOOOC

(-2200

F) for

50

hr.

mechanisms available for crack growth. Crack growth is relatively easy i

the

grain

boundaries

o

the

material

are

coated

with

a gl

ass

phase. At

high

temperature localized creep of

this

glass

can

occur resulting

in

grain

boundary sliding. Figure 8.8(a) shows the fracture surface of an

NC-132

hot pressed

Si

J

N

4

specimen

that fractured

after 2.2

min under

a static

bending load of 276 MPa (40,000 psi) al

- llOO C

(- 2000 F).

he

initial

flaw was probably a shallow (20

10

40

pm

machining crack.

t

linked up

with cracks formed

by grain boundary sliding

and

separation

and

pores

formed by triple-point cavitation

to

produce the new

Haw

or structurally

weakened region seen

in

Fig

8.8

as the large

semicircular

area

extending

inward from

the tensile surface. This was the effective

flaw

size at

fracture


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Time, Temperature, Environmental Effects on Properties 325

Figure 8.8 Comparison of a slow crack growth fracture versus a normal bend

fracture for hot-pre

ssed

Si

.

1

N From Ref. 9.)


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.-..

-.

.

..... ;

- -

.:. .:-: .

...

. -

.

: : : ; :;:

b

)

(a)

d)

(e)

Figure 8.11 Surfaces of hot-pressed Si)N. before and after oxidation. a) As_machined surface, 32O-gril diamo nd; b) oxidized

in air for 50 hr al 98O C l

8OO

F); c) oxidized

in

air fo r 24 hr at 12 ) rC (22OO F); and d) oxidized in air for 24 hr at 137O C

(25OO F) . C ASM International.)


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348

~

igure 8.23 Reaction-bonded

S i l ~

after exposure

in

a combustion rig with

5

ppm sea

salt addition for 25 cycles

of

1.5 hr at 900C 0.5 hr at

1120

C and a

5-min air-blast quench. a) , b), and c)

show the fracture surface

at

increasing

magnification and illustrate the glassy

buildup

in the

region

of

combustion gas

impingement. From Ref. 9.

Chapter 8

as fouling A thin buildup can protect

th

e surface from corrosion and

erosion and in some cases can even result in a local temperature reduction.

All three

of

these factors can increase the life of a component especially

a metal. However, a thick buildup reduces the airflow through the engine

and

decreases efficiency

Fouling is an inherent

problem

in

the direct burning

of coal. A variety

of

approaches have been or arc being studied to resolve this problem:

L Intermittent removal of buildup by thermal shock, melt-off,

or

passing abrasive material such as nutshells) through

the

system


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388

Chapter 9

Figure 9.4 Si.N

J

grinding media showi

ng

one of the common configurations.

Spheres are also commo

nl

y used. Courtesy KemaNord.)

wear-resistant linings and have

been

used successfully with dry milling and

with

water

as a milling fluid.

However

. some milling is conducted with

organic fluids that may att ack ru bber or polyurethane . Very

hard

grinding

media can reduce

contamination

because they wear more slow l

y w

is

goo

d for

some

cases

because

its high

hardness

reduces

wear

a nd its high

specific gravity minimizes milling tim

e.

f

contamination from the media

is a n especially critical

consideration

milling can be

conducted

with media

made of th e sa me compos ition as the powder being mill

ed.

Another ap-

proach

is to

mill with st

eel

media

a

nd

remove

the

contamination

by acid

leaching.

Milling can be

conduc

t

ed either

dry or wet. The

advantages

a

nd

dis-

adva

ntages

are listed

in

Table

9 5

Dry

milling has

the

adva

nt

age

that

the

resulting p

owde

r d

oes not have

to be

separa

ted from a liquid . The

major

concern

in

dry

milling is

that

the p

owde

r d

oes not

pack in the

corners of


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402

Chapter 9

Figure 9.10 Transmission electron microscope image o ultrafin L < i < I ~ r

powder prepared y the glycine-nitrate process. Courtesy o Larry Chick. Battelle

Northwest Laboratories, Richland . Wash .)


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422

Chapter

1

100

I m

Figure 10.3 Photo taken with a scanning electron microscope showing the spher

ical morphology

of

spray-dried powder. Courtesy Cerarnalec, Inc.)

closest possible packing; and (2) to minimi

ze

friction and allow all regions

of the compact to receive equivalent pressure. Let

us

discuss these in more

detail and examine some examples

inders and Plasticiz ers

Table 9.13 listed a variety

of

organic and inorganic materials that have

been

used

as binders . Most binders and pla

st

icizers are organic They

coat

the ceramic particles and provide lubrication during pressing and a tem

porar

y bond after pressing. The amount

of

organic binder required for

pressing

is

quite low, typically ranging from 0.5 to 5 wt

.

Organic binders

normally are decomposed during the high-temperature densification step

and evolved as gases. ome binders leave a carbon residue, especialty if

fired under reducing conditions.

Inorganic binders also exist. The clay minerals such as kaolinite are a

good example. Kaolinite has a layered structure and interacts with water


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Shape-Forming Processes

ressed

shape

owder

required

433

Figure

10_11

Schematic illustrating the different distances a punch must move to

accomplish uniform compaction

of

the powder. Based on a powder

wi

th

a com

paction ratio of 2: 1

10

ASM International.)

plastically during pressing and conforms to the contour of the die cavity.

The pressed shape usually contains flash (thin sheets of material at edges

where the material extruded between the die parts) and can deform after

pressing if not handled carefully. For these reasons, wet pressing

is

not

well-suited to automation. Also. dimensional tolerances are usually only

held to

: :2

.

Uniaxial ressing roblems

The following are some

of the

problems that can be encountered with

uniaxial pressing.

improper density or size

die wear

cracking

density variation.

The

first

two are easy to detect

by

simple measurements on the green

compact immediately after pressing. Improper density or size are often

associated with off-specification powder batches and are therefore relatively

easy to resolve.

ie

wear shows up as progressive change

in

dimensions.

t shou

ld

also be routinely handled

by

the process specification and quality

control.

he source of cracking may be more difficult to locate. t may be due

to improper die design.

air

entrapment, rebound during ejection from the

die. die-wall friction, die wear, or other causes. Often a crack initiates at

the top edge of the part during pressure release or ejection of the part.

Two mechanisms of this type cracking are illustrated

in

Fig. 10.13. he

first, shown in Fig. 1O.13(a), occurs as pressure is released from the upper


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434

o Powder

e Rig d Ole Parts

I Z2l Moving Ole Parts

Chapter

1

Figure 10 12

Schematic o tooling to uniaxially press a three-level part. ASM

International.)


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444

Chapter 10

Figure 10.21 Ceramic parts formed

y

uniaxial and isostatic pressing, some with

gree n machinin

g

(Co

urt esy Western

Gold

and Platinum

Company,

S

ub

sidiary

of

GTE Sylvania. Inc.

10.2 CASTING

When most people hea r the term casting. they automatically think

of

meta l casting in which a shape is for med by po uring molten metal into a

mo

ld

. A limited a

mount of

cas ting of molten ceramics is

done

in the

preparation

of

high-density p and AI,O

1

ZrO, refractories

and in

prep

ara tion

of

some abrasive materials . In the latter case, casting from a melt

into cool

ed

metal plates produces rapid quenching, which r

es

ults

in

very

fine crystal size that imparts high toughness to the materia

l

The technique

of cas ting molten ceramic refractories is called fusion cas ing.

More frequently. the casting of ceramics is done by a room-temperature

operation in w

hi

ch ceramic particles suspended in a liquid are cast into a

porous mold that removes the liquid and leaves a pa rt iculate compact in

the mo ld . T here are a num ber of vari a

ti

ons to this process. depending on

the viscosity

of

the ceramic-liquid suspension. the mold , and the procedures

used. The most common is refe rred to as slip casling. The principles and

contro ls for slip cas ting a re similar to those of the other particul ate ceramic


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-

u

i

0

u

0

:;

458

1000

C

100

70Wt. %

solids

0.05 0.1

0.2 0.

25

Volume % dispersant

R.glon A

MonazollneT

Monazoline-C

SedisperseO

Zany

FSN

MonazollneQ

Wilconol H31A

Flourad FC-170-C

Region B

Wileamlne PA78B

Monawet MM80

Aerosol AYl00

Aerosol

C 61

Monawel

MB

45

Manawa MO 70

OlsperslnotC

R,glone

Menhaden fish all

Emphos PS

- 2 A

ZonylA

Chapter 10

I

I AMP-95

I Alkazlne-TO

I Alkazlne-O

Emeras 2423

I Dispersinot-HP

I

SedlsperseF

I

I

I

Orewfax.()()7

I Aerosol.()T

I

Ouponol.a

pva

I Aerosol TR-70

I Amerlate LFA

I

Figure 10.32 Summary o the effect of the dispersants listed

in

Table 10.6 on the

viscosity

of slips

consisting

of

BaTiO) in

a MEK-ethanol so lvent.

Adapted from

Ref.

12_

These are referred to

as

non queous nonwater-ba sed). Another non

aqueous system utilizes trichloroethylene plus ethanol. Nonaqueous sys-

tems work well with steric hindrance because they are adequate solvents

for the chain polymers. Some of the polymers also provide ster

ic

hindrance

in

an queous water-based) system, for example, phosphate esters.

Aqueous slips utilizing electrostatic repulsion are commonly used for

slip casting. Techniques o slip preparation and slip casting are discussed

in

the following sections. Nonaqueous slips utilizing steric hindrance are

commonly used for tape casting. Tape casting is discussed later

in

this

chapter.

Slip reparation

The actual physical preparation of the slip can be done by a variety of

techniques. Perhaps the most common is wet ball milling or mixing.

he

ingredients. including the powder, binders. wetting agents, sintering aids,

and dispersing agents, are added to the mill with the proper proportion of

the selected casting liquid and milled to achieve thorough mixing, wetting,

and usua ll

y

particle size reduction. The

sl ip is

then allowed to age until


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ShapeForming Processes 465

Figure 10.36 Annular combustor for gas-turbin e engine fabricated by drain casting

using nonabsorbing pine and mandrel inserted into the mo

ld

. Courtesy Garrett

Turbine Engine Company, fabricated by Norton Company.)

channels into the stator vane mold during cast ing. The reservoir and vane

patterns are bonded together by simple

wax

welding and are shown

as

the

white wax assembly

in

the center

o

Fig. 10.37. Below this is the mold

produced by dipping and dissolving the pattern. Below the mold is the

green casting after dissolving

th

e mold and trimming off any material re

maining in the reservoir or gating area. The stator vane discussed above

required less than 1 hr casting time. Some solid castings require much

longer time such as the prototype gas-turbine rotor shown in Fig. 10.38.

t

required over 12 hr. The slip must be very stable for such long casting

time to avoid settling

o

large particles or adverse changes in viscosity.

Other fugitive mold techniques have been developed to fabricate spe

cial shapes. One technique produces low weight, but strong ceramic foam

19). Reticulated foam similar to a dishwashing sponge is used as the mold

interior. Ret iculated polymer foam

o

the desired pore size is cut to the

desired shape and placed in a container in a vacuum chamber. A ceramic

slip in poured into the container

and

under vacu

um

complete

ly

infiltrates

the pores in the reticulated foam. The slip is dried and fired to burn off

the polymer foam and densify the ceramic. The resulting part consists

o

an

internal cast

o

the spongelike foam. Its major characteristic is contin

uous interconnected links

o

ceramic and continuous pore channels. Such

a cellular structure can be very

li

ghtweight and surprisingly strong. Ex-


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66

Chapter 10

. .

1

.

Figure 10.37

The

fugitive-wax technique for

preparing

a complex-shape mold for

slip

castingj example of

fabrication

of a stator vane

for

a gas-turbine engine. Cour

tesy AiResearch Casting Company . Division

of

The

Garrett Corporation, presently

the

Garrett Ceramic

Components Division of Allied-Signal Aerospace.

amples are shown in Fig. 10.39. Note from the photograph that a variety

of

pore sizes have

been achieved from

several

different ceramic materials.

he

materials are successfully used for molten metal filtration and kiln

furniture and are being evaluated for removing particles from the exraust

of

diesel engines.

Some components are too complex to

e

fabricated in one piece by

casting. An example

is

th

turbine scroll shown in Fig. 10.40 a). The turbine

scroll

is

an important component

in

many gas-turbine designs.

t

changes

the direction of the hot gases coming out of the combustor to allow them

to pass through the rotor. The scroll in Fig. 10.40 a)

is

SiC. t was fabricated

by assembling the parts shown in Fig. JO.40 b) [20]. The shroud, sleeve ,

and ring were formed y isostatic pressing and green machining. The body

and duct were fabricated y slip casting. The parts were successfully bonded

together with a CrVTi braze developed at Oak Ridge National Laboratories

ORNL).

A final casting technique

is ele trophoreti deposition

EPD).

t

utilizes

an electrostatic charge to consolidate ceramic particles from a suspension.


467/844

Shape-Forming

Processes

67

cm

Figure 10.38 Prototype gas-turbine

rotor

fabricated by slip

casting

using a

fugitive

waxtype

process

, Courtesy AiResearch Casting

Company.

Division of

the

Garrett

Corporation. presently

the Garrett

Ceramic Components Division of AlliedSignaJ

Aerospace.

An electrical polarity

is

applied to the mold that

is

opposite to the polarity

at the surface of the ceramic particles The ceramic particles are electrically

attracted to the mold surface and deposit

as

a uniform compact When the

desired thickness o deposit is achieved, either the mold is removed from

the container of slip or the slip

is

poured from the mold Electrophoretic

deposition

is

generally used to deposit a thin coating or to produce a thin-

walled body such

as

a tube t is also used to achieve very uniform dep-

osition

o

spray paint onto a conductive surface.

All o the casting techniques discussed above result

in

a relatively weak

ceramic powder compact A technique recently developed at

ORNL

results

in

a much stronger compact This technique

is

referred to

as

gel casting

The ceramic powder

is

mixed with a liquid and a polymerizable additive

to form a fluid slurry similar to a casting slip The slip

is

poured into a

container o the desired shape. Polymerization is caused to occur before

the powder

in

the slip has time to settle The resulting powder compact

is

quite uniform and strong. However . removal o the liquid is more difficult

than for conventional slip casting. Furthermore, monomers are generally


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a)

b)

47


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472/844

472

Slurry

source

octor

blade

Warm

I

air

. /

( source

: = t ' ~ - -

' I ~

Take-up

\ ? ~ - - - - - - - - - - - - - . - - - - - - - - ~ ~ t : 1 reel

01

Reel

o

.....

.

carner

film

Supporl

structure

Chapter 1

Figure 10.41 Schematic illustrating the doctor blade tape-casting process,

Other

Tape Casting Processes

A second tapecasting process is the waterfall technique. It is iI1ustrated

in Fig. 10.42. The slurry is pumped in a recirculating system to form a

continuous

curtain.

A conveyor

belt

carries a flat

surface

through the slurry.

The uniform, thin layer of slurry on the carrier is then transferred

by

Slip

trough

Curta in 1

of

sl

ip

Drying stage

BBB

11l\\ /l \ X/l \\

Subs

t

ate

carrier

. ,

Conveyor

bell

c

C o f l ~ c t i o n

trough

Recirculating

pump

Figure 10.42 Schematic illustrating

the

wa terfa tape-casting proce

ss. (From

J

Adair

, University of Florida.)


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86

Chapter 1

Figure 10.52 A variety o ceramic

parts

that have been fabricated

by

extrusion.

Photo courtesy Superior Technical Ceramics Corporation, St. Albans, Vermont.)

pose and leave a carbon residue The acrylic binders are an exception

They burn out cleanly

in

inert and reducing atmospheres as well as oxidizing

a

tmospheres

.

ommon Extrusion Defects

Extrusion is often more of an art than a science Quality is controlled

y

careful inspection of extruded compacts for defects Defects that can occur


487/844

Shape-Forming Processes

87

for extrusion include

warpage

or distortion lamination tearing cracking

segregation, porosity, and inclusions

[27J

Warpage or distortion can occur during drying or firing

due to density

variations or during extrusion due to improper die alignment or die design.

f

the alignment or balance of the die

is

not correct, greater pressure on

one side of

the

die

will

occur.

This will

cause

more

material to extrude

from this

side

and result in

bending of the extruded

column as

it exits the

die.

aminations are cracks that

generally form a

pattern

or orientation.

Examples are

shown in Fig.

10.53. A

common

cause

is

incomplete re

knitting as the plastic

mix

is cut by the auger or flows past the spider portion

of the die. The spider

is

the portion of the die that supports any shaped

channels in the die. For example to extrude a

circular

tube a solid

rod

of the inner diameter of the tube must be supported at the center of the

die. t is generally supported by three prongs at 120 to each other that

run parallel to the length of the die and are attached to the inside of the

die. The material being extruded must squeeze around these prongs and

reunite into a continuous hollow cylinder before leaving the die.

Lami

nations occur if the material does not completely reknit.

earing consists of surface

cracks that

form

as the material

exits

the

extruder. This is illustrated

in

Fig. 10.54. The cracks extending from the

surface

inward

result

from

the contact stresses

and friction that are dis

cussed

earlier

in

this

chapter. Too dry a

mix

with inadequate cohesiveness

will

tear. A

mix

with high rebound may also tend to tear. Die design

involving a slight divergent

taper

at

the die

exit

can help

prevent

tearing.

Lamination and tearing

are

two

sources of cracking.

ther cracks can

occur due to

poor

mixing, shrinkage

variation

and partially dried debris

from a

prior

extrusion

run.

Segregation

involves a separation of

the liquid and

solid portions of

the

mix

during

extrusion.

This

can

result in cracking

or distortion

during

extrusion or during subsequent drying or

firing.

Figure 10.53 Drawings of the cross sections of extruded parts illustrating the

appearance

of severe laminations that can

occur

as

extrusion

defects. From Ref.

27.)


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Shape.Forming Protcsses

89

Figure 10.55 Cross sections of extruded honeycomb structures of cordierite for

use

as

catalyst supports for automotive emission-control devices. Courtesy NGK

Insulators.)


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s

Chapter 10

the

flow

pattern with alternate sprue and gate designs . In Fig. 10.59(a),

the gate

is

at the end, but

is

directed perpendicular to the length of the

cavity. In Fig. 1O.59(b), the gate

is

directed perpendicular, but placed at

the center of the mold cavity. Plug flow resulted in both cases and knit

line

formation

was minimized. This is

further

illustrated in

Fig

. 10.60 for

actual injectionmolding trials. The short shot technique was used

whereby injection

was

interrupted before the cavity

was

full By conducting

a sequence of short shots, a good image of the nature of mold fill for each

gate configuration could be obtained.

After binder removal

and

densification, knit lines remain as large

cracks, voids,

or

laminations and severely

limit

the strength of the part

The short shot approach has been successfully used at Carborundum

Company in developing integral radial rotors for an experimental auto

motive gas turbine [37

J

Initial rotors were injected from the nose end.

(Figure 10.61 illustrates the cross section of a radial rotor and identifies

terminology that will be referred to subsequently). Short shots indicated

a tendency for folds and knit lines to form in the thick region of the hub

near the backface. This

is

illustrated

in

Fig. 10.62. This region is exposed

to the

highest

stresses during

engine operation,

so

major iterative efforts

were conducted to minimize the

knit

line s Many parameters such

as

die

temperature, injection pressure, ho

ld

time, and sprue bushing /nozzle

di-

ameter

were

systemmatically varied

Sixteen

resulting

rotors were spin-

tested and failed at an average speed of 80,500 rpm, significantly below

Figure 10.60 Sequence o short shots showing the nature o mold fill for two

different sprue and gate orientations, t\ ASM International.)


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S 2

Chapter 1

(a)

Figure 10.62 Sequence of short shots for injection molding of a SiC

rotor

from

the nose end. Note the knit Jines in the hub and backface regions . (Photos courtesy

Carborundum Company for parts fabricated for Allison Gas Turbine Division of

General Motors under sponsorship of the U.S. Department of Energy and admin

istration of NASALewis Research

Center.)

noncrystalline to crystalline. For

exa

mple ,

the

volume change for

one

pol

ypropylene sys

tem due to

thermal contraction was about 2.75 vol

and

due to crystallization was about 1.75

vol

for a total

of

about 4.5 vol .

f the outer shell is rigid

and cannot

shrink, while the inner material is

more fluid and can reposition during further cooling, 4.5 shrinkage is

adequate

to form a void or crack through the

center

of the part. Such a

void or crack is typically not visible by examining the surface of the injec

tion-molded

part and

may not even be visible

after

densificiation . Figure

10.65 illust rates a large lenticular (lens-shaped) void

in

a

Si

, N, turbocharger

rotor

that

resulted primarily from th is mechanism.

pplicatiolls

o

Illjectioll Moldillg

Injection molding

is

usually selected for ceramics only

after

other

processes

have been rejected.

t

can produce a high degree of complexity. but the


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b)

e)


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5 4

Chapter 1

a)

Figure 10.63 Sequence of short shots for injection molding of a Si rotor from

the shaft end. Note the absence of knit lines in the hub and backface region.

Photos courte

sy arborundum ompany

for

parts fabricated

for Allison

Gas

Tur-

bine Division of General Motors under sponsorship of the U.S. Department of

Energy and administration of NASA-Lewis Research Center.)

initial cost of tooling is very high . For example, a mold to fabricate an

individual turbine blade can cost over 10,000 and a mold for a turbine

rotor over ]00,000. Molds for simple shapes and molds made o aluminum

for low-pressure injection molding aT much less expensive . s a result,

the use of injection molding of ceramics is increasing.

Injection molding is presently used to manufacture a variety of parts

including cores for investment lost-wax) casting

of

metals, weld caps,

thread guides, threaded fasteners (nut and bolt pairs), radomes, and pro-

totype gas-turbine engine components, Drawings of complex investment

casting cores for cooled metal gas-turbine blades or stator vanes are shown

in Fig, 10.66. During investment casting, the core is mounted in a ceramic

mold. Molten superalloy is poured into the mold around the core. The


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b)

e)


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506

Chapter 10

.

,

r ' I .

c

-..

1, ' .:

, . , ..

I

Figure 10.64 Examples o optimized SiC rotors injection-molded from the shaft

end. The rotor on the left

is

as-molded the one on the right

is

after sintering.

Photo courtesy Carborundum Company,)

ceramic mold

is removed from the outside of the

metal

part The

injection-

molded ceramic

ore

is leached from the interior

o

the blade or vane to

leave a complex cooling path This substantially reduces the cost o man

ufacturing

o

internally cooled stator vanes and rotor blades for advanced

gas-turbine engines.

Examples

o

o

david richerson modern ceramic engineering.pdf

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